A liposome is a closed vesicle comprising a lipid bimolecular film. It is considered that a natural biomembrane has a lipid bimolecular structure and thus liposomes have been widely used as a biomembrane model in studies on physicochemical properties of biomembranes. Furthermore, a number of substances can be enclosed in the internal aqueous layer or within the membrane of a liposome and then fused with a cell or incorporated into a cell. Thus liposomes have been used as a carrier for transporting substances to and into cells.
Attempts have been widely made to apply liposomes to various purposes in the fields of, for example, biology, medicine and pharmacology, so as to employ liposomes as a carrier for transporting enzymes or cancerocidal substances, to use liposomes for immunological purposes, to utilize the interaction of liposomes with cells or to apply liposomes as a drug delivery system.
Although liposomes are widely applicable to various purposes as described above, it has been recognized that liposomes have a brittle membrane structure, such that a chemical or physical change, in the lipids, constituting the membrane, causes some irreglularities in the orientation of the membrane. This brings about the leakage of the content of the liposomes, or the association or aggregation of liposomes with each other. As a result, a precipitate is formed.
In order to overcome this problem, a number of attempts to form a vesicle using synthetic amphiphatic compounds as analogs of naturally occurring phospholipids have been reported (refer to, for example, "Liposome," ed. by Nojima, Sunamoto and Inoue, Nankodo, Chap. 8). However none of the vesicles thus obtained is satisfactory as a drug carrier from the viewpoint of stability and lack of toxicity to the human body.
Known examples of an amphiphatic compound having an oligopeptide in the hydrophilic moiety and two long-chain alkyl groups in the hydrophobic moiety include those reported by Ihara et al. [Polym. Commun., 27, 282 (1986); Polymer J., 18, 463 (1986); Chem. Lett., (1984), 1713; and J. Jap. Chem., (1987), 543] and reported by Shimizu et al. (Chem. Lett., (1989), 1341; Thin Solid Films, 180 (1989), 179; JP-A-2-69498 and JP-A-2-71836] (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However none of these amphiphatic compounds is suitable as a drug carrier, since either it fails to form a vesicle of a monolayer or monolayer vesicle, if formed, is easily converted into other structures.
On the other hand, it is expected that molecular assemblies comprising a monolayer having a molecular orientation or multilayers, which are ultra-thin and dense, are widely applicable to materials for electronics devices and materials for protecting surfaces as well as to functional films for sensors based on the selective permeability of a gas molecule or an ion and permeation-controlling films for delivering materials.
The Langmuir-Blodgett ("LB") technique has been commonly known as a method for laminating a monolayer comprising amphiphatic compound molecules formed on a gas/liquid interface on a substrate. Recently, various LB films produced by this technique have been widely employed as organic ultra-thin films [refer to Kotai Butsuri, 17 (12), 45 (1982)].
Although molecular assemblies including LB films exert various functions based on the orientation of molecules and the ultra-thin properties, they have a highly delicate film structure from a physical standpoint and thus are liable to degradation or decomposition. It is observed in the cases of some of such compounds, furthermore, that the film structures suffer from many defects or irregularities and thus high density cannot be achieved. Therefore it is required for all uses to physically strengthen the film structures of these molecular assemblies to thereby provide uniform and highly dense films.
An effective means for physically strengthening the film structure of a molecular assembly is crosslinkage or polymerization of molecules.
Relating to the polymerization of, for example, LB films, conventional polymerizable compounds and polymerization modes are summarized by H. Bader et al. [Advances in Polymer Science, 64, 1 (1985)] and R. Buschl et al. [Macromol. Chem. Suppl., 6, 245 (1984)].
Polymerizable amphiphatic compounds have been frequently investigated. At the early stage of these studies, the major means employed comprised polymerizing unsaturated vinyl diene and diacetylene compounds, which were selected as polymerizable compounds, by irradiation using UV or rays such as .gamma.-rays. Although the polymers obtained by these methods had fast structures, the order of the molecular arrangement was poorly maintained after cleavage of unsaturated bonds.
As A. Laschewsky and H. Ringsfdorf [Macromolecules, 21, 1936 (1988)] point out, the number of well-ordered polymerizable compounds is very limited, because the orientation of a film is significantly affected by the length of an alkyl chain and the terminal hydrophilic group.
A. Laschewsky et al. further disclose [J. Am. Chem. Soc., 109, 788 (1987)] that polymerizable groups in various amphiphatic compounds having unsaturated bonds, which are useful in, for example, radiation polymerization, should be kept via spacer groups in order to maintain the order of molecular arrangement. Furthermore, JP-A-57-159506 shows an example of the application of a monolayer and multilayer polymeric film of an unsaturated compound (surfactant) produced via radiation polymerization as an ultrafiltration membrane.
Known techniques for polymerizing these compounds having unsaturated bonds via radiation suffer from the following problems. Namely, one problem resides in the fact that a specific molecular design strategy (for example, inserting a spacer group) is required in order to avoid some irregularities in the molecular orientation or irregular aggregation and precipitation of molecules caused by the polymerization. A second problem resides in the fact that the irradiation with UV or .gamma.-ray would frequently induce the decomposition or denaturation of various additives which are present together with the polymerizable amphiphatic compounds. A third problem resides in the fact that the film thus obtained via such a polymerization usually has extremely poor biocompatibility, which restricts the types and number of applications of such a product in living tissues to, for example, a permeation-controlling membrane for drugs.
Therefore, J. Am. Chem. Soc., 109, 4419 (1987) proposes a method for forming disulfide bond via the oxidative polymerization of dithiol without using radiation. Alternately, it is effective to radical-polymerize the above-mentioned compounds having unsaturated bonds in the presence of an initiator. In these methods, however, an initiator used at the polymerization needs to be removed from the film system after completion of the polymerization. In addition, the effects of the uses of an initiator involving redox agents on coexisting substances should be taken into consideration which initiator may have optimental effects on other components of such films.
Furthermore, the condensation polymerization of a molecular film of an amino acid derivative has been attempted in the presence of carbodiimide in order to improve the polymerization mode and enhance the biocompatibility [refer to J. A., Chem. Soc., 108, 487 (1986).] However this method cannot be easily performed too, since there is a problem of the residual condensing agent and by-products and it is required to control the efficiency of the condensation reaction.